This invention relates to apparatus for measuring the profile of a surface. In particular, but not exclusively, this invention relates to apparatus for measuring the profile of a surface projecting transversely of an axis of rotation of an object, for example the profile of the cam surface on a video drum cam. The cam surface of a video drum cam is in the form of a narrow ledge extending radially outwardly of the drum and defining a helical path around the drum. This cam surface serves to guide a video tape along a helical path past the video recording and/or reproducing heads within a video tape recorder or player.
Rank Taylor Hobson Limited of Leicester, United Kingdom manufacture metrology instruments designed to measure surface profiles. For example the Form Talysurf series 2 Laser Traverse Unit has been used to enable measurement of the overall form or shape of a cam surface. In such apparatus, a stylus is carried at one end of a support arm which is mounted so as to be pivotable about a pivot axis to allow the stylus to follow the surface during relative movement between the stylus and the surface. The displacement of the stylus as the stylus follows the surface is detected using a transducer and the output of the transducer is logged together with signals indicating the position of the tip of the stylus along its line of travel to provide data giving a measurement of the surface profile.
An article by A. Frank entitled xe2x80x9cNanometric Surface Metrology at the National Physics Laboratoryxe2x80x9d published in Nanotechnology 11 (1991) at pages 11-18 describes a stylus laser profilometer wherein the surface to be measured is mounted on a carriage or slideway and a pivotally mounted stylus follows the profile of the surface as the slideway is moved relative to the stylus. The displacement of the stylus is measured by means of a double-pass interferometer comprising a roof prism which is cemented to the stylus so as to form the moving member of the interferometer. The static part of the interferometer is mounted directly above the roof prism and is rigidly referenced to the slideway.
In use of such apparatus, as the stylus follows the surface profile, the stylus inevitably moves through an arc of a circle centred on the pivot axis of the support arm. The result of this is a slight displacement of the stylus in the direction of movement of the carriage as the stylus pivots about the pivot axis. This slight displacement means that the measurement point is somewhat displaced from that which was intended and this may introduce errors into the measurement results. In addition, if such apparatus is used to follow a narrow surface or ledge such as the cam surface of a video drum cam, there is a possibility that the stylus may lose contact completely with the surface being measured.
In a first aspect, the present invention provides a metrological apparatus having a stylus carried by a support arm arranged to pivot about a pivot axis to allow the stylus to follow a surface during relative movement between the stylus and the surface and means for maintaining the support arm at a substantially constant attitude.
In a second aspect, the present invention provides apparatus for measuring the profile or shape of a surface of an object positioned so that the surface projects transversely of an axis about which relative rotation of a stylus and the surface is effected, the stylus being carried by. a support arm which is pivotable about a pivot axis to allow the stylus to follow the profile of the surface during relative rotation of the stylus and the surface, and means being provided for controlling the support arm to maintain the support arm at a substantially constant attitude.
In another aspect, the present invention provides a metrological apparatus comprising a support arm for carrying at one end a stylus, the support arm being arranged to pivot about a pivot axis to allow the stylus to follow a surface during relative movement between the stylus and the surface and means for maintaining the support arm at a substantially constant attitude.
In an embodiment of the present invention, inaccuracies in measurements resulting from the pivotal movement of the stylus support arm may be avoided or at least reduced because the support arm is maintained at a substantially constant attitude avoiding or at least reducing any displacement of the stylus due to pivotal movement of the support arm, thereby enabling an accurate determination of the location of the point being measured by the stylus. In addition, where the surface being measured is quite narrow, for example as in the case of a cam surface on a video cassette recorder drum cam, maintenance of the stylus on the relatively narrow surface or ledge may be facilitated.
Relative rotation may be effected by use of a turntable to support the object. It may, however, also be possible to mount the stylus support arm so that it is the stylus rather than the object which rotates about the axis of rotation.
Drive means may be used to maintain the support arm at a substantially constant attitude which may be, for example, horizontal or an angle to the horizontal. The actual angle at which the drive means maintains the support arm may be determined in accordance with the manufacturing tolerances within the apparatus and in accordance with the characteristics, for example, the inclination of the surface being measured so as to reduce measurement errors. Means may be provided f or adjusting the attitude of the support arm.
Movement of the stylus as it follows a surface may be measured by measuring means which may comprise an interferometer, although it may be possible to use an inductive means such as an LVDT (linear variable differential transducer).
A measuring element of the measuring means may be mounted, as close as is practically possible so as to be aligned with a tip of the stylus so that the measuring element provides a direct indication of the movement of the stylus without being affected by pivotal movement of the support arm. For example, where the measuring means comprises an interferometer, then the measuring element may comprise a retroreflector which is aligned with the tip of the stylus. As an example, the retroreflector may comprise a corner cube having its apex aligned with the stylus tip.
To maintain the support arm at a substantially constant attitude, an LVDT having one of its core and coil, generally the core, coupled to the support arm and feedback means for driving the support arm to maintain a predetermined output from the LVDT may be used. As another possibility, an optical transducer may be used in place of the LVDT.
In another aspect, the present invention provides measurement apparatus having a number of selectable stylus support members each defining a different separation between a stylus tip and a measuring element.